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Riprap Stone Size & Scour Protection Calculator

Riprap rock revetments protect earth channels, river banks, and bridge piers from destructive hydrodynamic shear stresses caused by flood currents.

Maximum design flow velocity impinging on the embankment.

Rock density relative to water (standard granite / basalt is 2.65).

Empirical Isbash constant reflecting turbulence intensity.

Design safety multiplier (typically 1.2 to 1.5).

Calculated Result
24.2 in

Median Stone Size (D₅₀)

Median Stone Diameter (D₅₀)

0.61 m (24.2 in)

Estimated Stone Weight (W₅₀)

321.4 kg (709 lbs)

Riprap Revetment Class

Class 4 / Armored Wave-Break Riprap (D50 ≥ 24 in / 600 mm, high energy scour zones)

Design Channel Velocity

3.5 m/s

Safety Factor

1.2:1

Calculation Breakdown

  1. Isbash Stone Stability FormulationD₅₀ = 1.2 × [3.5 / (0.86 × √(2 × 9.81 × (2.65 - 1)))]² = 0.61 m
  2. Equivalent Stone MassW₅₀ = (π / 6) × (2.65 × 1000 kg/m³) × (0.61 m)³ = 321.4 kg

Riprap Design Parameters

Interactive visualization based on your current inputs

Value
0.021416282D50 (inches)D50 (cm)W50 (kg)Velocity (m/s × 10)ParameterValue

What Is the Riprap Stone Size & Scour Protection Calculator?

The Riprap Stone Size & Scour Protection Calculator sizes blasted rock revetments to prevent soil erosion along rivers, bridge abutments, and storm outlet spillways.

How Does the Riprap Stone Size & Scour Protection Calculator Work?

Computes the minimum stable stone size where submerged rock weight overcomes hydrodynamic shear stress and turbulent drag.

Riprap Stone Size & Scour Protection Calculator Formula & Variables

The core mathematical equation utilized by this calculator is expressed as:

D_{50} = SF \cdot \left[ \frac{V}{C \sqrt{2 g (S_s - 1)}} \right]^2, \quad W_{50} = \frac{\pi}{6} \rho_{rock} D_{50}^3

Isbash equation balancing hydrodynamic lift and drag against submerged stone gravity.

How to Use the Riprap Stone Size & Scour Protection Calculator

  1. Enter design peak velocity at channel bed or embankment.
  2. Select rock specific gravity and channel turbulence regime.
  3. Read median stone diameter D50, stone weight, and standardized riprap class.

Step-by-Step Example Calculation

River Bend Revetment Armouring

Input Values:

channelVelocityMs:3.5
specificGravityStone:2.65
isbashConstant:high-turbulence-bend-spillway
safetyFactor:1.2
Worked Steps: Yields D50 = 15.3 in (390 mm, Class 3 Heavy Riprap) with individual median stone mass of 82 kg.

Understanding Your Result

Median Size D50: Sieve diameter where 50% of the rock mix by weight is smaller.

Median Weight W50: Weight of individual median stone.

Riprap Class: Light, standard, heavy, or armored specification classification.

Factors That Affect the Result

  • Stone stability scales with velocity squared (V²); doubling water speed quadruples required stone diameter.

When Should You Use This Calculator?

  • Culvert outlet aprons, bridge scour counter-measures, river training dikes, and coastal sea wall revetments.

Assumptions & Limitations

  • Assumes angular blasted rock; rounded river boulders require a 25% diameter increase.

Frequently Asked Questions

Calculation Accuracy & Reference Note

Standard USACE EM 1110-2-1601 and FHWA HEC-23 methodology.

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